1. Field of the Invention
The present invention relates to communications systems. More particularly, the present invention relates to a method and apparatus for performing hard handoff between different wireless communication systems.
2. Description of the Related Art
In a code division multiple access (CDMA) spread spectrum communication system, a common frequency band is used for communication with all base stations within that system. An example of such a system is described in the TIA/EIA Interim Standard IS-95-A entitled “Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System”, incorporated herein by reference. The generation and receipt of CDMA signals is disclosed in U.S. Pat. No. 4,901,307 entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEMS USING SATELLITE OR TERRESTRIAL REPEATERS” and in U.S. Pat. No. 5,103,459 entitled “SYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM” both of which are assigned to the assignees of the present invention and incorporated herein by reference.
Signals occupying the common frequency band are discriminated at the receiving station by using a high rate pseudonoise (PN) code. The PN code modulates signals transmitted from the base stations and the mobile stations. Signals from different base stations can be separately received at the receiving station by discrimination of the unique time offset that is introduced in the PN codes assigned to each base station. The high rate PN modulation also allows the receiving station to receive a signal from a single transmission station where the signal has traveled from the base station to the receiving station over several different distinct propagation paths (commonly referred to as “multipathing”). Demodulation of multipath signals is disclosed in U.S. Pat. No. 5,490,165 entitled “DEMODULATION ELEMENT ASSIGNMENT IN A SYSTEM CAPABLE OF RECEIVING MULTIPLE SIGNALS” and in U.S. Pat. No. 5,109,390 entitled “DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM”, both of which are assigned to the assignee of the present invention and incorporated herein by reference.
The use by all base stations within a particular system of a common frequency band allows simultaneous communication between a mobile station and more than one base station. This is commonly referred to as “soft handoff”. One implementation of a soft handoff method and apparatus is disclosed in U.S. Pat. No. 5,101,501 entitled “SOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM” and U.S. Pat. No. 5,267,261 entitled “MOBILE STATION ASSISTED SOFT HANDOFF IN A CDMA CELLULAR COMMUNICATIONS SYSTEM”, both assigned to the assignee of the present invention and incorporated herein by reference. Similarly, a mobile station can be simultaneously communicating with two sectors of the same base station, known as “softer handoff” as disclosed in U.S. Patent Application entitled “METHOD AND APPARATUS FOR PERFORMING HANDOFF BETWEEN SECTORS OF A COMMON BASE STATION”, U.S. Pat. No. 5,625,876 issued Apr. 29, 1997, assigned to the assignee of the present invention and incorporated herein by reference. An important feature is that both soft handoffs and softer handoffs make the new connection before breaking the existing one.
If a mobile station travels outside the boundary of the system with which it is currently communicating, it is desirable to maintain the communication link by transferring the call to a neighboring system, if one exists. The neighboring system may use any wireless technology, examples of which are CDMA, NAMPS, advanced mobile phone service (AMPS), time division multiple access (TDMA), or global mobile systems (GSM). If the neighboring system uses CDMA on the same frequency band as the current system, an inter-system soft handoff can be performed. In situations where inter-system soft handoff is not available, the communication link is transferred through a hard handoff where the current connection is broken before a new one is made. Examples of typical hard handoff situations include: (1) the situation in which a mobile station is traveling from region serviced by a CDMA system to a region serviced by a system employing an alternate technology and (2) the situation in which a call is transferred between two CDMA systems which use different frequency bands (inter-frequency hard handoff).
Inter-frequency hard handoffs can also occur between base stations of the same CDMA system. For example, a region of high demand such as a dense urban area may require a greater number of frequencies to service demand than the suburban region surrounding it. It may not be cost effective to deploy all available frequencies throughout the system. A call originating on a frequency deployed only in the high congestion area must be handed off as the user travels to a less congested area. Another example is a system that encounters interference from another service operating on an interfering frequency within the system's boundaries. As users travel into an area suffering from interference from another service, their call may need to be handed off to a different frequency.
Handoffs can be initiated using a variety of techniques. Handoff techniques, including those using signal quality measurements to initiate a handoff, are found in U.S. Patent Application entitled “METHOD AND APPARATUS FOR HANDOFF BETWEEN DIFFERENT CELLULAR COMMUNICATIONS SYSTEMS”, now U.S. Pat. No. 5,697,055 issued Dec. 9, 1997, assigned to the assignee of the present invention and incorporated herein by reference. Further disclosure of handoffs, including measurement of round-trip signal delay to initiate handoff, is disclosed in U.S. Patent Application entitled “METHOD AND APPARATUS FOR HARD HANDOFF IN A CDMA SYSTEM”, now U.S. Pat. No. 5,848,063 issued Dec. 8, 1998, assigned to the assignee of the present invention and incorporated herein by reference. Handoffs from CDMA systems to alternate technology systems are disclosed in U.S. Patent Application entitled “METHOD AND APPARATUS FOR MOBILE UNIT ASSISTED CDMA TO ALTERNATIVE SYSTEM HARD HANDOFF”, U.S. Pat. No. 5,594,718, issued Jan. 14, 1997, assigned to the assignee of the present invention and incorporated herein by reference. In the '718 patent, pilot beacons are placed at the boundaries of the system. These beacons are transmitted within the frequency band that is being monitored by the mobile that approaches, allowing the mobile to monitor the pilot beacon without retuning to another frequency band. When a mobile station reports these pilot beacons to the base station, the base station knows that the mobile station is approaching the boundary, and in response, prepares for the possibility of an intersystem hard handoff.
When a system has determined that a call should be transferred to another system via hard handoff, a message is sent to the mobile station directing it to do so along with parameters that enable the mobile station to connect with the destination system. The system from which the mobile station is departing has only estimates of the mobile station's actual location and environment, so the parameters sent to the mobile station are not guaranteed to be accurate. For example, with beacon aided handoff, the measurement of the pilot beacon's signal strength can be a valid trigger for the handoff. However, those base stations in the destination system that can effectively communicate with the mobile station are not necessarily known. However, those base stations with which the mobile has effectively communicated and which are considered to be good candidates based upon additional criteria are maintained in a list within the mobile station. Inclusion in the list is based upon allocation of forward link resources by the base station in question. Allocation of forward link resources by all possible candidate base stations is wasteful of system resources and reduces available system capacity, since only a relatively few candidates are typically required.
One method for increasing the probability of successfully completing a hard handoff is disclosed in U.S. Patent Application entitled “METHOD AND APPARATUS FOR PERFORMING MOBILE ASSISTED HARD HANDOFF BETWEEN COMMUNICATION SYSTEMS”, U.S. Pat. No. 5,999,816 issued Dec. 7, 1999, assigned to the assignee of the present invention and incorporated herein by reference. In most current systems, the mobile station has only one radio frequency (RF) front end circuit. Therefore, only one frequency band can be received at a time. Therefore, in order for the mobile station to communicate with the destination system, contact with the origination system must cease. In the '746 application, mobile stations tune temporarily to the frequency of the hard handoff destination system and search for available pilot signals (hereafter referred to simply as “pilots”) on that frequency, for inclusion of the associated base stations in the active set. After the searching task is completed, the mobile station will retune to the original frequency to resume current communications. While tuned to an alternate frequency, any frames of data generated by the mobile station or transmitted by the base station will be corrupted. Typically, the base station will provide only a subset of the possible offsets (commonly referred to as an “enable list”) for the mobile station to search.
A method for minimizing the duration of the search is disclosed in a U.S. Patent Application entitled “METHOD AND APPARATUS FOR PERFORMING MOBILE STATION ASSISTED HARD HANDOFF USING OFF LINE SEARCHING”, U.S. Pat. No. 6,134,440, issued Oct. 17, 2000. In this application, which is assigned to the assignee of the present invention and which is incorporated herein by reference, the receiver stores information received on the frequency band used by a potential hard handoff candidate base station. This information is not processed until after the receiver is tuned back to the frequency band used by the origination base station. By storing the information for processing after the receiver has retuned to the frequency used by the origination base station, the receiver can be tuned to the frequency of the origination base station more of the time. Thus, less information is lost. Nonetheless, when the origination base station is transmitted at relatively high data rates, information will be lost. When such information is lost, the base station must retransmit the information, or the receiver must make due without that information. Accordingly, there is a need for a method and apparatus that further reduces the amount of information that is lost while tuning to alternate frequencies, such as when attempting to identify potential hard handoff candidates.